This invention relates generally to the field of electrochemical cells, and in particular to flow field plate arrangements for membrane electrode assemblies in fuel cells.
Fuel cells are electrochemical devices which directly combine hydrogen from a fuel and oxygen, usually from the air, to produce electricity and water. With prior processing, a wide range of fuels, including hydrogen, natural gas, methanol, gasoline and coal-derived synthetic fuels, can be converted to electric power. Fuel cells may be used as stationary electric power plants in buildings and residences, as vehicle power sources in cars, buses and trucks and as portable power in video cameras, computers and the like. The basic process is highly efficient (40-75%), pollution-free, quiet, free from moving parts (other than an air compressor, cooling fans, pumps and actuators) and may be constructed to leave only heat and water as by-products. Since single fuel cells can be assembled into stacks of varying sizes, systems can be designed to produce a wide range of energy output levels and thus satisfy numerous kinds of applications.
There are several different types of fuel cells under such labels as phosphoric acid, alkaline, molten carbonate, solid oxide and proton exchange membrane (PEM). The basic components of a PEM fuel cell are the two electrodes separated by a polymer membrane electrolyte. Each electrode is coated on one side with a thin catalyst layer. The electrodes, catalyst and membrane together form a membrane electrode assembly (MEA). In a typical PEM-type fuel cell, the MEA is sandwiched between “anode” and “cathode” diffusion mediums (hereinafter “DMs”) that can be formed from a resilient and conductive material such as carbon fabric or paper. The anode and cathode DMs serve as porous electrical conductors between catalyzed sites of the PEM and the fuel (e.g., hydrogen) and oxidant (e.g., air/oxygen) which flow in respective “anode” and “cathode” flow field plates.
Channels formed in the flow field plates supply hydrogen and air to the electrodes on either side of the PEM. In particular, hydrogen flows through the channels to the anode where the catalyst promotes separation into protons and electrons. On the opposite side of the PEM, air flows through the channels to the cathode where oxygen in the air attracts the hydrogen protons through the PEM. Electrons are captured as useful electricity through an external circuit and are combined with the protons and oxygen to produce water vapor at the cathode side.
The channels forming the flow field plates have a cross sectional width and a land separating adjacent channels. The pitch of a flow field plate is the cross sectional width of the channel plus the cross sectional width of an adjacent land. Prior to the present invention, all flow field plates have utilized the same pitch on both the anode and cathode sides. As such, it has been necessary to provide land-to-land alignment across the MEA so that the compression loads can reduce the electrical contact resistance. Further, it has been desirable with such prior art flow field plates to provide as much land-to-land contact as possible so that the compression stress (force per unit contact area) is reduced to avoid localized damage to the diffusion media (DM) and MEA. However, to achieve good cell performance, it is desirable to have narrow lands to provide a minimum diffusion distance. However, narrow lands require precise land-to-land alignment which can be difficult to achieve due to manufacturing and assembly tolerances.
Accordingly, the present inventors have recognized a need to improve the design of the fluid flow plates of the fuel cells in order to increase cell performance.